The present invention relates to a non-aqueous electrolyte secondary battery (hereinafter, battery), and especially relates to batteries whose electrochemical properties such as the charge/discharge capacity and charge/discharge cycle life have been enhanced by improvements in negative electrode materials, and solvents used for the non-aqueous electrolytes.
In recent years, lithium secondary batteries with non-aqueous electrolytes, which are used in such fields as mobile communications devices including portable information terminals and portable electronic devices, main power sources of portable electronic devices, small size domestic portable electricity storing devices, motor cycles using an electric motor as a driving source, electric cars and hybrid electric cars, have characteristics of a high electromotive force and a high energy density.
When lithium metal with a high capacity is used as a negative electrode material, dendritic deposits are formed on the negative electrode during charging. Over repeated charging and discharging, these dendritic deposits penetrate through separators and polymer gel electrolytes to the positive electrode side, causing an internal short circuit. During discharging, these dendritic deposits break, falling from the surface of the bulk lithium-metal negative electrode, thus forming xe2x80x9cdeadxe2x80x9d lithium which does not contribute to charge/discharge reaction. Furthermore, reaction activity of the deposited lithium is high since they have a large specific surface area. Due to this, the lithium reacts with solvents in the electrolytic solution on their surfaces, and form a surface film similar to a solid electrolyte which has no electronic conductivity. This increases the internal resistance of the batteries, causing some particles to be excluded from the network of the electronic conduction, thereby lowering the charge/discharge efficiency of the battery. For these reasons, the lithium secondary batteries using lithium metal as a negative electrode material have a low reliability and a short cycle life.
To suppress the formation of such dendrites, it has been proposed that lithium alloys such as a lithium-aluminum alloy and a wood""s alloy are used instead of lithium metal. Metals capable of forming alloys with lithium and alloys containing at least one such metal can be used as a negative electrode material with a relatively high electrochemical capacity in the initial charge/discharge cycle. However, by repeatedly alloying with and deintercalating lithium, they may undergo a phase change even when the crystal structure of the original alloy is maintained, or sometimes, the crystal structure of the alloy itself changes.
In this case, particles of the metal or an alloy which are host materials of the lithium (active material), swell and shrink. As the charge/discharge cycle proceeds, crystal grains are stressed and cracked, thus particles are pulverized and leave off from the electrode plate. As the particles are pulverized, grain boundary resistance and contact resistance of the grain boundaries increase. As a result, resistance polarization during charging and discharging increases. Thus, when charging is conducted at a controlled voltage level, charging depth becomes shallow, limiting the amount of charged electricity in the battery. On the other hand, during discharging, the voltage level is decreased by the resistance polarization, reaching the discharge-termination voltage early. Thus, superior charge/discharge capacity and cycle properties are difficult to achieve.
Nowadays, lithium secondary batteries which use, as a negative electrode material, carbon materials capable of intercalating and deintercalating lithium ions, are commercially available. In general, lithium metal does not deposit on carbon negative electrodes. Thus, short circuits are not caused by dendrites. However, the theoretical capacity of graphite which is one of the currently used carbon materials is 372 mAh/g, only one tenth of that of pure Li metal.
Other active material compounds include diniobium pentaoxide (Nb2O5), titanium disulfide (TiS2), molybdenum dioxide (MoO2), lithium titanate (Li4/3Ti5/3O4). In the case of these materials, lithium is ionized and maintained among the host substances. Due to this, compared with lithium metal whose chemical activity is high, these materials are chemically stable, do not form dendritic deposits, and contribute to higher cycle properties. Among them, some carbon materials are already commercialized.
Other known, negative electrode materials include pure metallic materials and pure non-metallic materials which form composites with lithium. For example, composition formulae of compounds of tin(Sn), silicon (Si) and zinc (Zn) with the maximum amount of lithium are respectively Li22Sn5, Li22Si5, and LiZn. Within the range of these composition formulae, metallic lithium does not normally deposit. Thus, an internal short circuit is not caused by dendrites. Furthermore, the electrochemical capacities between these compounds and each element mentioned above are respectively 993 mAh/g, 4199 mAh/g and 410 mAh/g; all are larger than the theoretical capacity of graphite.
As other compound negative electrode materials, the Japanese Patent Laid-Open Publication No. H07-240201 discloses a non-metallic silicide comprising transition elements. The Japanese Patent Laid-Open Publication No. H09-63651 discloses negative electrode materials which are made of inter-metallic compounds comprising at least one of group 4B elements, P and Sb, and have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
As a solvent of the electrolyte of the battery, cyclic carbonates such as propylene carbonate and ethylene carbonate, acyclic carbonates such as diethyl carbonate, and dimethyl carbonate, cyclic carboxylate such as gamma-butyrolactone and gamma-valerolactone, acyclic ethers such as dimethoxy ethane and 1,3-dimethoxy propane, and cyclic ethers such as tetrahydrofuran and 1,3-dioxolane are widely used.
It is desirable to adopt electrolyte with high electrical conductivity to the batteries. Due to this, solvents with a high dielectric constant and a low viscosity are preferably used. However, being high in the dielectric constant simply means high in polarity, in other words, high in viscosity. Therefore, among the electrolytes mentioned above, solvents with high dielectric constant such as propylene carbonate (dielectric constant ∈=65) and solvents with low dielectric constant such as 1,2-dimethoxy ethane (∈=7.2) are often mixed and used.
The electrolyte used in the non-aqueous electrolyte batteries also contain supporting electrolytes dissolved in the solvents mentioned above at a concentration of about 1 mol. The supporting electrolytes include anion lithium salts of inorganic acid such as lithium perchlorate, lithium borofluorides and lithium phosphofluoride, and anion lithium salts of organic acid such as trifluoromethane sulfonic acid lithium and bis-trifluoromethane sulfonic acid imido lithium.
But, the above high capacity negative electrode materials include following problems.
Negative electrode materials of pure metallic materials and pure non-metallic materials which form compounds with lithium have inferior charge/discharge cycle properties compared with carbon negative electrode materials. The reason for this is assumed to be the destruction of the negative electrode materials caused by their volume expansion and shrinkage.
On the other hand, as negative electrode materials with an improved cycle life property unlike the foregoing pure materials, the Japanese Patent Laid-Open Publication No. H07-240201 and the Japanese Patent Laid-Open Publication No. H09-63651 respectively disclose non-metallic silicides composed of transition elements and intermetallic compounds which are composed of at least one of group 4B elements, P and Sb, and have a crystal structure of one of the CaF2 type, the ZnS type and the AlLiSi type.
Batteries using the negative electrode materials comprising the non-metallic silicides composed of transition elements are disclosed in the Japanese Patent Laid-Open Publication No. H07-240201. The capacities of the embodiments of the invention and a comparative example at the first cycle, the fiftieth cycle and the hundredth cycle suggest that the batteries of the invention have improved charge/discharge cycle properties compared with lithium metal negative electrode materials. However, when compared with a negative electrode material of natural graphite, the increase in the capacity of the battery is only about 12%.
The materials disclosed in the Japanese Patent Laid-Open Publication No. H09-63651 have a better charge/discharge cycle property than a Li-Pb alloy negative electrode material according to an embodiment and a comparative example. The materials also have a larger capacity compared with a graphite negative electrode material. However, the discharge capacity decreases significantly up to the 10-20th charge/discharge cycles. Even when Mg2Sn, which is considered to be better than any of the other materials, is used, the discharge capacity decreases to approximately 70% of the initial capacity after about the 20th cycle. Thus, their charge/discharge properties are inferior.
When lithium metal is used as a negative electrode, the electrolyte, which is in contact with the negative electrode, and is exposed to an extremely high reduction atmosphere, tends to react with the lithium metal, and is consequently reduced, and decomposed. Regarding lithium alloys, when those predominantly composed of lithium are used in the negative electrode, the potential of the negative electrode becomes almost same as that of lithium metal, thus reduction and decomposition of the electrolyte occur in the same manner as the lithium metal. Furthermore, as mentioned earlier, the negative active materials get pulverized over repeated charges and discharges, and inevitably fall off from the electrode plate.
In the case of the alloys whose main constituent metal is not lithium, the potential of the negative electrode becomes noble compared with lithium metal or the foregoing lithium alloys. Thus, the electrolyte, which could be reduced and decomposed when contacting the foregoing lithium alloys, can be used. However, compared with the foregoing lithium alloys, these alloys whose main constituent metal is not lithium are hard and brittle, and thus get pulverized significantly, and inevitably fall off from the electrode plate.
If currently used solvents such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, propylene carbonate, gamma-butyrolactone, and gamma-valerolactone are adopted for a system in which lithium metal or a lithium alloy is used in a negative electrode, the electrolyte may decompose and gas may be produced when the battery is charged and stored at high temperatures. Moreover, if the battery is repeatedly charged and discharged, parallel to the charge/discharge reaction in the negative electrode, the electrolyte is gasified, lowering the charge/discharge efficiency, resulting in decreased cycle properties.
When graphite-group carbon materials are used as a negative electrode material, and propylene carbonate is adopted for an electrolytic solution, the electrolyte decomposes at potentials more noble than that of lithium metal. Consequently, lithium ions are not intercalated between layers of graphite, thus the battery does not function. Considering these points, currently commercialized lithium secondary batteries with the graphite used for negative electrode materials frequently use electrolyte containing ethylene carbonate. However, the melting point of ethylene carbonate is 37xc2x0 C. higher than room temperature. Therefore, at low temperatures, ionic conductivity of the electrolyte for lithium ions plummets, lowering charge/discharge priorities.
When inorganic compound materials such as titanium disulfide are used as a negative electrode active material, intercalation and de-intercalation of lithium occur at sufficiently noble potentials compared with lithium metal and lithium alloys. Thus, even when the negative electrode active materials come into contact with the electrolyte, reduction decomposition does not occur. Moreover, even when propylene carbonate is used for the electrolytic solution, intercalation and de-intercalation are not impeded by decomposition as it is the case with the graphite materials, therefore, a wider range of electrolytes are applicable. However, potentials of the negative electrode using the foregoing inorganic compound materials is noble, causing battery voltage to inevitably become low. This is a disadvantage of achieving higher energy density.
Regarding the supporting electrolytes, the thermal stability of lithium perchlorate, lithium borofluorides and lithium fluorophosphate needs to be improved. Furthermore, fluorine-containing inorganic anion salts contained in the forgoing compounds react with trace amounts of water contained in an electrolyte and decompose.
The present invention aims to address the forgoing problems of conventional batteries.
The negative electrode of the batteries of the present invention is characterized by its main material which uses composite particles constructed in such a manner that at least part of the surrounding surface of nuclear particles containing at least one of tin, silicon and zinc as a constituent element, is coated with a solid solution or an inter-metallic compound composed of an element contained in the nuclear particles and at least one element (exclusive of the elements contained in the nuclear particles) selected from a group comprising group 2 elements, transition elements, group 12 elements, group 13 elements and group 14 elements (exclusive of carbon) of the Periodic Table.
Further, the solvents of the electrolyte of the batteries of the present invention include at least one compound selected from a group comprising ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, propylene carbonate, gamma-butyrolactone and gamma-valerolactone.
Moreover, the supporting electrolytes in the electrolyte of the batteries of the present invention includes at least one compound selected from a group comprising bis-trifluoromethane sulfonic acid imido lithium, bis-pentafluoro ethane sulfonic acid imido lithium, bis(1,2-benzene diolate(2-)-O,Oxe2x80x2)lithium borate, bis(2,3-naphthalene diolate(2-)-O,Oxe2x80x2)lithium borate, bis(2,2xe2x80x2-biphenyl diolate(2-)-O,Oxe2x80x2)lithium borate, and bis(5-fluoro-2-olate-1-benzene sulfonic acid-O,Oxe2x80x2)lithium borate.
The foregoing construction provides batteries which rarely suffer generation of gas when stored at high temperatures. Moreover, even when the batteries are repeatedly charged and discharged, charge/discharge efficiency of their negative electrode does not decrease. The batteries can be used in a wide range of temperatures. Furthermore, the batteries enjoy high energy density, maintain discharge capacity well over repeated use as well as high charge/discharge properties.